Fine-tuning the pore environment of ultramicroporous three-dimensional covalent organic frameworks for efficient one-step ethylene purification

The construction of functional three-dimensional covalent organic frameworks (3D COFs) for gas separation, specifically for the efficient removal of ethane (C2H6) from ethylene (C2H4), is significant but challenging due to their similar physicochemical properties. In this study, we demonstrate fine-tuning the pore environment of ultramicroporous 3D COFs to achieve efficient one-step C2H4 purification. By choosing our previously reported 3D-TPB-COF-H as a reference material, we rationally design and synthesize an isostructural 3D COF (3D-TPP-COF) containing pyridine units. Impressively, compared with 3D-TPB-COF-H, 3D-TPP-COF exhibits both high C2H6 adsorption capacity (110.4 cm3 g−1 at 293 K and 1 bar) and good C2H6/C2H4 selectivity (1.8), due to the formation of additional C-H···N interactions between pyridine groups and C2H6. To our knowledge, this performance surpasses all other reported COFs and is even comparable to some benchmark porous materials. In addition, dynamic breakthrough experiments reveal that 3D-TPP-COF can be used as a robust absorbent to produce high-purity C2H4 directly from a C2H6/C2H4 mixture. This study provides important guidance for the rational design of 3D COFs for efficient gas separation.


Supplementary Instruments
1 H and 13 C NMR spectra were measured on a Bruker Fourier 400 MHz spectrometer.
High-resolution mass spectra were collected on Bruker Solarix.Elemental analysis was conducted on a Flash EA 1112.Fourier transform infrared (FTIR) spectra were recorded on a Nicolet iN10 micro FTIR Spectrometer.Powder X-ray diffraction (PXRD) patterns were obtained on a Rigaku SmartLab X-Ray diffractometer with Cu Kα line focused radiation at 45 kV and 200 mA or Rigaku MiniFlex 600 X-Ray diffractometer.
Thermogravimetric analysis (TGA) from 30-800 °C was carried out on a TA-Q500 in nitrogen atmosphere using a 10 °C/min ramp without equilibration delay.Fieldemission scanning electron microscope (FE-SEM) was performed on a ZEISS SIGMA operating at an accelerating voltage ranging from 0.1 to 20 kV.The 13 C CP MAS spectra were recorded on a Bruker AVANCE NEO 400 WB spectrometer equipped with a 4 mm standard bore.The CP MAS probe head whose X channel was tuned to 100.62 MHz for 13 C and the other channel was tuned to 400.18 MHz for broad band 1 H decoupling, using a magnetic field of 9.39 T at 297 K.The dried and finely powdered sample was packed in the ZrO2 rotor closed with Kel-F cap which was spun at 8 kHz rate.The experiments were conducted at a contact time of 2 ms.A total of 2000 scans were recorded with 3 s recycle delay for each sample.All 13 C CP MAS chemical shifts are referenced to the resonances of adamantane standard.UV-vis spectra were recorded on a SHIMADZU UV-3600 UV-vis-NIR spectrophotometer.
The nitrogen sorption isotherms were measured at 77 K by using an Autosorb-iQ2 (Quantachrome) surface area size analyzer.Before measurement, the samples were degassed in a vacuum at 120 °C for 12 h.The Brunauer-Emmett-Teller (BET) surface area was calculated from selected points and pore size distribution was calculated with the NLDFT method.

Synthesis
After filtration, the precipitate was washed with water and dried in vacuo to obtain a brown solid (22 g, 83 %).To a solution of the brown solid (10 g, 37.5 mmol) in 48 % HBr (30 mL) was added dropwise a saturated aqueous solution of NaNO2 (20.7 g, 300 mmol) at 0 °C.The reaction system was then stirred at room temperature for 4 h.The pH of the solution was adjusted to 8 by the addition of aqueous NaOH solution, and the resulting mixture was extracted with ethyl acetate.The organic solution was washed with water, brine, and dried over anhydrous Na2SO4.After that, the solvent was evaporated under reduced pressure and the crude product was purified by column chromatography [SiO2: heptane/dichloromethane = 5/1] to yield the 2,3,5,6tetrabromopyridine as a white solid (1.3 g, 20 %). 1 H NMR (400 MHz, CDCl3, ppm): δ = 7.99 (s, 1 H).

Synthesis of tetra(p-aminophenyl) methane (TAPM)
TAPM was synthesized based on the literature. 2 To a mixture of tetra(pnitrophenyl)methane (1.5 g, 2.99 mmol), Raney-nickel (10 g) and THF (100 mL), hydrazine monohydrate (10.0 g) were added dropwise and refluxed for 3 hours.After cooling to room temperature, the mixture was filtered and solvent was evaporated under reduced pressure.The crude product was washed with ethanol and dried to obtain TAPM as a white solid (0.7 g, 61% yield). 1

E. Thermogravimetric Analysis
Supplementary Figure 5 TGA profile of the 3D-TPP-COF in nitrogen atmosphere.

F. Chemical Stability
In a general process, 10 mg of 3D-TPP-COF was immersed into different solvents (DMF, DMSO, 1,4-dioxane, ethanol, toluene, acetonitrile, 10 -2 M HCl and 12 M NaOH) for 24 h at room temperature.Then the powder was filtrated, washed with tetrahydrofuran and dichloromethane and dried under vacuum to measure PXRD patterns.Data were collected on Rigaku MiniFlex 600 X-Ray diffractometer.
Supplementary Figure 6 PXRD patterns of 3D-TPP-COF before and after being treated in different solvents for 24 h.

Section 2. Crystal structure analysis
The method of cRED data collection was similar with our previous report. 3,4Shortly, the sample was cooled down to 99 K by using Gatan cryo-transfer tomography holder and the data was collected by a quad hybrid pixel detector (Timepix) with video mode, using the software of instamatic. 5All the ED patterns were recorded under the spot size 3 with the exposure time 0.5 s.Data processing was conducted using the software package XDS 6 and REDp. 7Structure solutions were performed by SHELXT 8 with the merged and scaled datasets.PXRD patterns were obtained on a Rigaku SmartLab X-Ray diffractometer with Cu Kα line (λ = 1.5418Å) focused radiation at 45 kV and 200 mA.The Rietveld refinement with rigid-body constraints was performed on the experimental PXRD using Topas 5. 9 Supplementary Table 1 Fractional atomic coordinates and the unit cell of 3D-TPP-COF from the Rietveld refinement.Supplementary Figure 14 The adsorption enthalpies (Qst) of C2H6 (red) and C2H4 (blue) for 3D-TPP-COF.

Table 4
The interactions between the gas molecules and host framework in 3D-TPB-COF-H and 3D-TPP-COF-B.